Clutter, or more specifically ground clutter, is the general designation of signal contamination in the form of echoes or reflections that a remote sensing instrument, such as a weather radar for example, gets from trivial, unwanted, stationary targets. A major task of a weather radar is to measure the reflectivity and speed (relative to the radar station) of airborne precipitation, i.e. raindrops and ice particles, in which task strong echoes from stationary obstacles are a nuisance. A number of signal processing approaches have been developed to remove ground clutter, while simultaneously salvaging as much of the actual measurement signal as possible.
A stationary reflector does not cause any phase shift between consecutive reflected pulses. Consequently a widely used approach is to examine the signal in the frequency domain (i.e. after performing an FFT) and to filter out those parts of the frequency spectrum that correspond to velocities at or near 0 m/s, either by deleting them completely or by interpolating over them with the help of data from the neighboring velocity bins. However, not all ground clutter comes from exactly stationary objects. For example a forest through which a wind blows contains a large number of individual reflectors, the velocities of which form a certain distribution around zero. Even a perfectly stationary reflector gives rise to a distribution in the frequency domain because the antenna rotates, so objects come into and go out of view.
The frequency domain approach works reasonably well with conventional pulse radars, but not with a more sophisticated one with multiple pulse repetition times. A radar of the last-mentioned kind is commonly referred to as a Multi-PRT radar, where PRT means pulse repetition time. The advantage of Multi-PRT is its far greater velocity range: with Multi-PRT it is possible to measure all naturally occurring wind speeds, while a conventional single-PRT pulse radar with a reasonable pulse repetition frequency can only measure up to 6 m/s in a C-Band radar. However, the cost is the modest signal-to-noise ratio of a Multi-PRT radar in the frequency domain. A single-PRT radar that employs the conventional approach for ground clutter removal may detect a rain signal that has a power of 50 dB below the power of the reflections from the ground, but an FFT calculated from the received signal of a Multi-PRT radar will contain a multitude of artifacts and noise at about 25 dB below the power level of the ground-reflected signal. This would mean that the Multi-PRT radar could only recover from at most 25 dB of ground clutter, which for many applications is not enough.
The industry standard clutter removal algorithm GMAP (Gaussian model adaptive processing) by Sigmet Inc. (U.S. Pat. No. 7,589,666 B2) performs poorly with Multi-PRT radar as it operates in the frequency domain. It also does not benefit from the information provided by modern dual-polarization weather radar. The US-American national weather service has upgraded their weather radar network to dual-polarization from 2011 to 2013. (http://www.roc.noaa.gov/wsr88d/dualpol/DualPolOverview.aspx)